Capacitive screen is a mainstream screen at current stage, and is widely used in the field of mobile terminal such as a mobile phone or a tablet personal computer. The projected capacitive screen type of the capacitive screen includes a self-capacitance screen and a mutual-capacitance screen. Taking a mutual-capacitance screen as an example, the structure of the mutual-capacitance screen includes a number of sending terminals TXs and a number of receiving terminals RXs. When touch detection is performed to the mutual-capacitance screen or the mutual-capacitance is detected for obtaining its magnitude, a sending terminal sends a scanning waveform of a certain magnitude and frequency. The scanning waveform may be a sine wave, a square wave, a trapezoidal wave and the like. The magnitude of the signal received by the receiving terminal will be adjusted by a demodulator, and the magnitude is tend to be affected by a mutual-capacitance at an intersection point of a sending terminal and a receiving terminal, and the magnitude of the mutual-capacitance reflects the position of the touch.
When the scanning waveform at the sending terminal is mixed with an interference signal such as an environmental noise, a RF noise or an impulse noise, the interference signal will be passed to the receiving terminal and affect the judgment of change in the amplitude of the signal at the receiving terminal, which leads to an inaccurate test result and affects the user experience of the capacitive screen. A signal of a fixed frequency such as a direct current signal may be output at the receiving terminal after demodulation of a normal excitation signal. But an alternating current signal of a certain frequency will be formed after demodulation of the interference signal. In order to filter out the alternating current signal, a noise reduction method in the detection of capacitive screen generally performs the filtering before and after the demodulation. The performance of the filtering determines the effect of the noise reduction.
Some of the existing filters are usually designed to have an accumulator structure. It is easier to implement the filtering function, since the accumulator requires only an addition unit. The performance of the accumulator depends on the duration for the accumulation. According to the results of frequency spectrum characteristic analysis of the structure of the accumulator in a low-pass filter, a cutoff frequency and high frequency attenuation are compressed in proportion to the increment of the duration for accumulation. That is to say the performance of the filter will be improved as the duration for accumulation being extended. For filters of other structures, a filter with a narrower bandwidth can achieve better noise reduction performance. However the filter may output stability data only after a period of time delay so as to achieve the function of filtering, and the narrower the bandwidth of the filter, the longer the time delay. Therefore, the filter may not be able to achieve the desired filtering effect if the amount of the input signals is relatively little, thereby the reliability of the output signal is affected. Based on the above case, according to the existing noise reduction method, the performance of noise reduction can be improved by increasing the number of the scan channels, i.e. by increasing the amount of the signals participated in the filtering process. However, since the length of the scanning waveform and the length of the scanning frame are limited, extending the scanning time at the sending terminal is limited, and therefore, increasing the amount of the signals is also limited. Moreover, since it is needed to control processes including the scanning and the demodulation simultaneously for increasing the number of the scan channels, there will be a problem that the design complexity of the sending terminal and the receiving terminal is increased.